NSRRC Activity Report 2023

Environmental and Earth Sciences 075 transformation of HAp to Cd-HAp in BC chunks. In particular, XND was used to obtain high-resolution XRD data at a spatial resolution higher than 100 nm × 100 nm × 50 nm, to confirm the mineral transformation and 3-D matrix scale incorporation of Cd into HAp. Thereafter, XRF associated with XND was used to perform elemental analyses and mapping on section samples. Transmission X-ray microscopy (TXM, TLS 01B1 ) was used to explore nano-level spatial distribution of porosity and refilling of porous space by Cd incorporation in BC. To the best of the group’s knowledge, no direct observation of 3-D incorporation of Cd and spatial distribution of Cd-related minerals in the BC mineral matrix via an ion-exchange mechanism has been documented to date. This is the first principal observation documenting the 3-D uptake of Cd and spatial distribution of Cd-HAp in the BC mineral matrix. 5 The Cd removal from aqueous solution and incorporation into the mineral lattice increased for 500BC compared to 700BC. The diffusion depth was modulated by the initial Cd concentration and charring temperature. A higher carbonate level of BC, more pre-leached Ca sites, and external phosphorus input enhanced Cd removal. The XND line scan revealed that Cd was primarily removed from water by incorporation into the mineral lattice of 500BC via ion exchange, rather than surface sorption and precipitation, and the mineral phase was transformed from HAp to Cd-HAp ( Fig. 2 ). Rietveld’s refinement of the XRD data resolved up to 91% of the crystal displacement of Ca 2+ by Cd 2+ , and most of the HAp was converted to Cd- HAp. TXM images revealed in situ sub-micron pore space refilling throughout the mineral matrix, indicative of Cd incorporation. 500BC showed a higher CO 3 2- /PO 4 3- ratio and specific surface area compared to 700BC, providing more vacant sites due to dissolution of Ca 2+ . The specific phase and stoichiometry of the newly formed Cd-HAp mineral was dependent on the level of ion exchange. This study highlights the potential role of BC in changing the environmental fate of Cd in retention and transport, and underpins the integration of Cd on a micro-scale into a 3-D mineral matrix of BC, which could later be removed from the field. In summary, the study revealed that the BC chunks prepared at 500 °C can act as a high-capacity and sustainable remediation material via a 3-D matrix scale ion- exchange mechanism and formation of Cd-HAp. 500BC is therefore an excellent material for heavy metal removal and field application even under weathering environments Fig. 2 : The mineralogical transformation patterns of HAp in 500BC after Cd incorporation. X-ray nanodiffraction detected Cd-HAp formed by ion exchange. [Reproduced from Ref. 5] (a) (c) (b) (d)